1. Field of the Invention
The invention relates to a resistive thin film. Such films are devoted for instance to make strain gauges that are used for high accuracy and very low power consumption and in particular to make pressure sensors.
The invention is also devoted to a measuring cell including a diaphragm with such a thin film mechanically linked to it and to a shell of a pressure sensor.
2. Description of the Prior Art
A pressure sensor module generally comprises a sensor shell having an aperture for receiving a fluid, the pressure of which is to be measured by the sensor. The shell comprises a separating membrane which can be deflected when a positive or negative pressure is applied by the fluid present at the aperture of the shell. The separating membrane tightly separates an inner closed volume of the shell from an open volume of the shell having the aperture. The closed volume of the shell contains a pressure transmitting medium and a measuring cell. The pressure of the fluid present in the open volume of the shell in then indirectly applied to the measuring cell, by means of the membrane and of the transmitting medium, for instance silicon oil. Because of the separating membrane the cell is immerged in the pressure transmitting medium and is then protected from corrosive fluids the pressure of which is to be measured. The measuring cell is made of two half cells bonded to one another. This hydrostatic configuration allows encapsulating the cell without rigid or mechanical bounding. This allows decoupling unwanted stress or staining resulting from, for instance, thermal expansion or mechanical shocks.
A hallow part inside the measuring cell allows for a deflection of a diaphragm of the cell. Resistors forming a bridge are mechanically linked, for instance bonded or deposited on a surface of a diaphragm of the measuring cell. When the diaphragm of the cell having the resistors linked on it is deflected by the pressure of the transmitting medium, the resistors are deformed. The deformation results in a variation of the value of the resistance of the resistors. The resistors may be made of thin film which are bonded or coated onto the diaphragm. For such an application the thin film must have high sensitivity to strain. This sensitivity is measured by a coefficient G call gauge factor which is given by the following formula:
  G  =            (                        ⅆ          R                          ⅆ          l                    )        *          1      R      
Where R and dR are respectively the value of the resistance and a variation of it, dl is a variation of a length l of the resistor.
In general, for metallic film (Platinum, Tantalum, Nickel, . . . ). the guage factor is purely geometrical and is around 2. For doped material such as for silicon with Bohr, the gauge factor can reach 80. For thin film composed of a microscopic mixture of metal grains and ceramic matrix (cermet), at the percolation (the mixture is around 50%-50%) the guage factor can reach 10.
A kind of thin film which is used for the purpose of making resistors, in particular resistors to be used in pressure cells is made of a cermet of tantalum (Ta) grains in a dielectric nitride of tantalum (NTa). These thin films have the particularity to have a high gauge factor, low temperature coefficient of resistance (TCR) and are very stable at high temperature. Document EP 0 526 290, incorporated herewith by reference, describes a process of preparation of such a thin film
To reduce sensor power consumption, for instance, for remote control and memory gauge purposes, the resistor must be as high as possible (several KOhm).
The shape of each resistor is generally chosen so that the length of the resistor is the longest that can fit with the surface of the diaphragm.
In the pressure bridge configuration, the resistance of diagonally opposed legs varies equally and in the same direction as a function of the mechanical deformation caused by pressure. As the resistance of on set of diagnally opposed legs increases under pressure, the resistance of the other set decreases. Bridge excitation in the form of a voltage or currency is applied across two opposite nodes of the bridge. These nodes are usually referred to as excitation inputs or bridge drive inputs. The bridge inputs are connected through tight connections to external connections of the measuring cell and then on external connections of the shell.